It is known to provide image display systems in which image elements, whether they are representing a two-dimensional image or a three-dimensional image, are displayed to a user with a selection and modified processing of at least some of the image elements being made in dependence upon the display values for those image elements. In certain cases, these image elements are directly related to a physical characteristic of an object through the measurement of signal strength values from a defection device. As an example, in medical imaging applications signal values representing the signals returned from CAT scanning or MRI scanning may be displayed with display elements having their intensity or colour controlled by the value of the signal returned for each particular image element. In order to improve the ease of interpretation of such images it is known to map different colours to different ranges of display value such that particular features, e.g. blood vessels, may be made more visible within the image.
A particular problem arises when it is desired to selectively process or display those image elements falling within a range of display values that is itself located within a broader full range of display values for the image as a whole. FIG. 1 of the accompanying drawings illustrates such an image. The image contains a region 2 of image elements having high display values, such as a region of bone within a medical image. This region 2 having high display values is located within an overall background 4 of image elements having low display values, such as image elements representing soft tissue. Also present within this image is a blood vessel 6 containing a contrast enhancing agent that is represented by image elements having display values located somewhere between the high values of the bone region 2 and the low values of the background soft tissue region 4.
FIG. 2 of the accompanying drawings illustrates the problem that occurs due to the finite resolution of real imaging systems in that at the interface between the bone region 2 and the soft tissue 4 there is a boundary formed of pixels having values lying somewhere between the high values of the bone region 2 and the low values of the soft tissue region 4. In many cases, these boundary pixels will have display values closely similar to the display values corresponding to the blood vessel 6. Accordingly, if the image of FIG. 2 is processed to enhance the appearance, or otherwise select in some way, the display elements having display values corresponding to the blood vessels 6, then this will erroneously also highlight a region at the interface between the bone region 2 and the soft tissue 4 that due to aliasing effects appears to have the appropriate display value.